Aluminum coating of ferrous metal and resulting product



Oct. 16, 1962 E. J. METS 3,058,206

ALUMINUM COATING OF FERROUS METAL AND RESULTING PRODUCT Filed Dec. 2'7,1956 r ALUMINUM CONTAINING 1-57. IRON IRON-ALUMINUM ALLOY .1; \rFERROUSBASE METAL.

United rates 3,058,206 ALUMINUM COATING F FERRQUS METAL AND RESUITDJGPRUDUCT Edwin J. Mats, Pittsiieid, Mass, assignor to General ElectricCompany, a corporation of New York Filed Dec. 27, 1956, Ser. No. 630,831Ciaims. (Cl. 29196.2)

The present invention relates to coating of ferrous metal articles, andmore particularly to a process of aluminum coating ferrous articleswhich results in improved corrosion resistance of the coated articles.

The advantages of a protective aluminum coating for ferrous articles arewell known in the art, including, for example, such benefits as goodadherence to the base metal, permanence in air due to formation of aninert oxide film, light weight, ductility, hardness and other desirablecharacteristics. However, While ferrous articles treated with knownaluminum coating processes may have, in general, satisfactory corrosionresistance properties under ordinary atmospheric conditions, it has beenobserved that known types of aluminum coated articles under prolongedexposure to sulfur-bearing atmospheres such as found in industrial areasor to salt-containing air such as prevalent in marine or coastal regionsexhibit marked corrosion effects which lead to ultimate deterioration ofthe ferrous article.

It is an object of the present invention to provide aluminum-coatedferrous base materials and articles having improved corrosionresistance, particularly under prolonged exposure to severely corrosiveatmospheres.

It is another object of the invention to provide an improved process ofproducing a protective aluminum coating on ferrous articles whichconfers increased corrosion resistance to the ferrous articles.

It is a further object of the invention to provide on ferrous base metalan aluminum coating which not only inherently is corrosion resistant butalso serves to minimize the electrochemical differences in the coatingstructure which would otherwise lead to corrosion effects.

The improved corrosion resistant ferrous product is produced, inaccordance with the invention, by immersing the ferrous article in abath of molten aluminum to which has been added a minor but effectiveamount of iron. It has been found that such addition of iron to thealuminum dipping bath markedly improves the protection afforded by thealuminum to the ferrous base material against corrosive agents, andparticularly weathering elfects in corrosive industrial atmospheres.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawing in which the singleFIGURE is a photornicrograph showing a cross section of a ferrousarticle coated in accordance with the present invention.

Various methods have been used in the past for alu minizing ferrousmaterials, and of these the so-called hotdip process, wherein theferrous article is immersed in a molten aluminum bath, has been foundquite practical and has been extensively used in industry. In thisprocess, when the article comes into contact with th molten aluminum inthe bath, an alloy bonding layer composed of iron-aluminum compounds isformed at the surface of the ferrous article. This continuous alloylayer is formed by diffusion of the molten aluminum into the base ironproducing compounds of definite chemical composition. These compoundsgenerally have a high iron content of the order of 60-70%, and whilethey do exhibit resistance to corrosion by high temperature oxidation,they show only slight resistance to corrosion under normal atmosphericconditions. In the usual hot-dip process, however, there is additionallyformed overlying the bonding alloy layer a distinct outer layer whichhas 3,058,206 Patented Oct. 16, 1962 a composition corresponding to thatof the aluminum bath. Since heretofore in the art reliance has beenplaced on the outer aluminum layer for elfective corrosion resistance,it has been the practice to use a coating bath of commercially purealuminum to form a pure aluminum outer layer overlying the intermediate(interfacial) alloy layer.

it has been found, in accordance with the invention, that by adding ironin the small amounts hereinafter specified to the molten aluminum bath,there is provided an outer layer of aluminum which contains thecorresponding amount of iron, and which considerably improves thecorrosion resistance of the coated ferrous article over that afiorded bythe relatively pure aluminum coatings heretofore used.

Shown in the drawing is a photomicrograph at 250x magnification of across section of a ferrous body treated in a hot-dip process inaccordance with the invention wherein the iron body was introduced intoa molten aluminum bath of commercially pure aluminum to which 1-5 ironby weight had been added. As illustrated, the coated structure comprisesthe iron base material 1, an interfacial bonding layer 2 ofiron-aluminum alloy compounds, and an outer layer 3 of an aluminum alloycontaining 1-5 iron.

As will be observed, the hot-dip coating process by which theillustrated structure was formed results in a composite coatingcomprising bonding layer 2 and outer aluminum alloy layer 3, each ofwhich constitutes a distinct homogeneous continuous coating overlyingthe base ferrous metal. In articles coated in the prior hotdipaluminizing processes, reliance was placed principally on the thicknessand density of these outer and intermediate layers to provide thenecessary protection against corrosion of the underlying base metal. Thethicker these coatings, the longer was the period of time requiredbefore external corrosive agents could penetrate the layers to the baseferrous metal. However, the ductility of the coatings decreased withincreased thickness and the necessity for maintaining sufficientductility to withstand the bending and forming operations to which theferrous articles were subjected limited in practice the thickness of theouter and intermediate layers. By virtue of the provision of the outeraluminum-iron alloy layer in accordance with the present invention,improved corrosion resistance can be obtained without the necessity foran excessively thick coating and without thereby sacrificing desirableductility in the coated product.

In a series of comparative tests made in connection with the presentdevelopment, a set of test panels of low carbon steel were coated inaluminum coating baths containing, respectively, varying concentrationsof Fe, Si, Zn and Ni. The steel panels thus coated were subjected toaccelerated corrosion tests comprising a standard salt fog exposure test(ASTM Spec. B117 54T) and sulfurbean'ng conditions simulating heavyindustrial atmospheres. As a result of these exposure tests, it wasfound that the test panels coated in aluminum baths containing Zn, Si,and Ni, and which varied in concentrations of those elements from showedconsiderably poorer corrosion resistance than panels coated withcommercially pure aluminum.

On the other hand, those samples coated from aluminum baths containingiron Within the limited range of about 1-5% showed significantimprovement in corrosion resistance in these tests. For example, aftermore than 2,000 hours in the salt fog exposure test, panels coated withcommercially pure aluminum (containing .45 Fe) showed characteristicblack etching and scattered red rust on 35% of their surface, whereaspanels coated with aluminum containing about 3.5% Fe showed nosignificant attack on the coating. In the same test, the panels having1.8% Fe in the aluminum coating showed etched black areas on about 20%of the surface, but no red rusting, panels with 2.6% Fe had lightscattered grey areas on about 10% of the surface, and panels with 5.3%Fe (removed after 921 hours exposure) exhibited an etched black attackon 80% of their surface accompanied by red rusting.

Analogous results were obtained in the sulfur exposure test, whereinafter 123 days exposure characteristic black spotting and red rustpitting effects were observed over the entire surface of panels coatedwith commercially pure aluminum and those coated with aluminumcontaining over 5% Fe. Least amounts of these corrosion effects wereobserved in the panels coated with 1.8% Fe, 2.6% Fe and 3.5% Fe, with nored resting and only a few scattered black spots being present in the3.5% Fe coating.

The results of these and other tests appeared to demonstrate than anamount of iron in the aluminum bath ranging from l-5%, and preferablyabout 3-4%, served to markedly improve the corrosion resistance of thealuminized articles under severe atmospheric conditions.

While it is not fully clear What produces this substantial improvement,some explanation may be found in fact that incorporating iron in theouter exposed layer reduces the electrochemical potential between theouter layer and the intermeditae bonding layer which consistsprincipally of FeAl compounds. This effect appears to be demonstrated ina test similar to those above using copper alloy nuts and bolts toconnect the aluminized panels, wherein less galvanic attack of thecoating was noted with the higher iron content panels than with those oflower iron content. So far as can be determined, there is no effectproduced by the iron addition on the Fe-Al compound intermediate layer.The increase in iron content apparently makes the outer layer morecompatible with the intermediate layer and the base iron, resulting in amuch slower rate of corrosion attack by galvanic action of the coatingas a whole. The improved effect does not appear to be influenced by bathtemperature or immersion time, but is dependent only on the compositionof the outer layer.

The role of the iron in the outer layer in producing im proved corrosionresistance in accordance with the invention is unexpected, in that theprior art has heretofore considered the presence of as much as 1% ironin aluminum to be detrimental to corrosion resistance, and efforts havegenerally been made in the past for that reason to use as pure aluminumas possible in the coating bath. For this reason also, it has been thepractice heretofore to avoid prolonged use of the coating bath, whichafter repeated dipping of ferrous articles tended to have a higher ironcontent. The present discovery, therefore, is of advantage in thatlonger alnminizing operations can now be carried on before the bath mustbe scrapped due to excessive iron content. It further appears thataddition of iron in the stated concentration tends to retard ftn'thersolution of iron from the base material into the bath.

It is important to note in this connection that merely immersing theferrous part for a longer period in the aluminum bath or raising thetemperature of the bath, while effecting increased removal of iron fromthe article, does not appreciably change the aluminum bath composition,since such steps merely serve to produce thicker intermediate alloylayers which utilize the additional iron removed from the base material.The distinct outer aluminum layer formed by the hot-dip process appearsto be constituted solely by the molten bath material clinging to thepart as it is removed from the bath, and in accordance with theinvention the composition of this outer aluminum layer is modified bythe addition of iron to the aluminum bath independently 'of any ironremoved from the base material during the dipping process.

A typical process which maybe used in practicing the invention is asfollows, it being understood that the particular procedure set forth ismerely illustrative andin no way limits the scope of the invention:

The ferrous article to be aluminized is initially vapor degreased andthen pickled in hot acid. After being rinsed in cold water, the part iscompletely dried. The thus cleaned and dried part is then immersed in amolten aluminum bath containing about 3% by Weight of iron, thetemperature of the bath being 700800 C. and the immersion period rangingfrom 30 seconds to about 4 minutes depending on the size of thearticles. The part is then removed from the bath and the coating thereonis allowed to solidify. After solidification of the coating the coatedpart is water quenched or air cooled.

The iron added to the molten aluminum bath may be in the form of softiron wire which dissolves in the bath, but it will be understood thatthe particular form of the iron added or the manner of addition is notcritical to the invention.

The thickness of the intermediate alloy layer 2 is preferably about 2-3mils, but as previously indicated the thickness can be controlled asdesired merely by varying the time of immersion and/or temperature ofthe bath. The thickness of the outer layer 3 in a typically coated partis about 2 mils, and this can be controlled to a practical extent byvariations in the viscosity of the aluminum bath.

Addition to the aluminum bath of amounts of iron after reaching themaximum 5% specified in accordance with the invention appears to cause arather abrupt decrease in corrosion resistance afforded by the outerlayer, whereas the addition of less than 1% iron produces very littleimprovement in resistance to'the galvanic corrosion efiectscharacteristic of the prior types of aluminized parts coated withcommercially pure aluminum.

In addition to the previously mentioned advantages of the presentinvention with respect to improved resistance to severely corrosiveatmospheres, and the longer use of aluminum dipping baths which itpermits, there is afforded wider latitude and greater economy in thetype of starting aluminum ingot material which may be used for coatingpurposes, since the cheaper grades of aluminum with iron content greaterthan the .25.5% requirements for commercially pure aluminum and withinthe 1-5 range of the present invention can be employed for aluminizingoperations.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. The method of coating a ferrous metal body which comprisesintroducing the ferrous metal body into a molten aluminum bath havingabout 15% iron incorporated therein, and removing the ferrous metal bodyfrom the bath.

2. The method of coating a ferrous metal body which comprises applyingthereto molten metal coating material consisting essentially of aluminumand about 15% iron.

3. In the method of hot-dip coating of ferrous base metal with aluminum,the step comprising adding about 34% iron to the molten aluminum bathbefore dipping the ferrous base metal therein.

4. A corrosion resistant product comprising a ferrous metal base, anintermediate layer of iron-aluminum alloy overlying said ferrous metalbase, and an outer layer composed of an alloy of aluminum and about 15iron overlying said intermediate iron-aluminum alloy layer.

5. A ferrous product having high resistance to corrosion in severelycorrosive atmospheres comprising a ferrous metal base, and a compositeprotective coating overlying the ferrous metal base, said compositeprotective coating comprising an inner layer of iron-aluminum alloycompounds and an outer layer composed of an alloy of aluminum and about34% iron.

6. A corrosion resistant product comprising a ferrous metal base, anintermediate layer of iron-aluminum alloy overlying said ferrous metalbase and having a thickness of about 2 to 3 mills, and an outer layerabout 2 mils thick composed of an alloy of aluminum and about 1-5 ironoverlying said intermediate iron-aluminum alloy layer and bonded therebyto said ferrous metal base.

7. A corrosion resistant product comprising a ferrous metal base, anintermediate layer of iron-aluminum alloy having about 60-70% ironoverlying said ferrous metal base and having a thickness of about 2 to 3mils, and an outer layer about 2 mils thick composed of an alloy ofaluminum and about 1-5% iron overlying said intermediate iron-aluminumalloy layer and bonded thereby to said ferrous metal base.

8. A corrosion resistant product produced by introducing a ferrous metalbody into a molten aluminum bath having about l5% iron incorporatedtherein, and removing the ferrous metal body from the bath.

9. The method of coating a ferrous metal body which comprises providinga molten bath composed of molten aluminum having about 1-5% ironincorporated therein, introducing a ferrous metal body into said moltenbath, and removing the ferrous metal body from the bath.

10. The method of coating a ferrous metal body which 2 comprises addingabout 15% iron to a molten aluminum bath, introducing a ferrous metalbody into the thus provided molten bath, and removing the ferrous metalbody from the bath.

References Cited in the file of this patent UNITED STATES PATENTS

4. A CORROSION RESISTANT PRODUCT COMPRISING A FERROUS METAL BASE, ANINTERMEDIATE LAYER OF IRON-ALUMINUM ALLOY